The present invention relates to an engine exhaust after-treatment device for trapping particulate matter and, in particular, relates to a diesel particulate trap constructed from a silicon carbide cermet foam for trapping the particulate matter. Combustion of the collected particulate cleans and regenerates the trap.
It is necessary in engine exhausts, and in particular diesel engine exhausts, to collect and trap particulate matter which is in the exhaust. The most commonly used filters for these traps are of the ceramic type (cordierite) and these filters are very effective in removing and trapping the particulate matter. However, the volume of the particulates trapped and removed from the exhaust is of such an amount that the filters have to be removed and replaced frequently.
In order to avoid removing or replacing the filters, prior art techniques burn the collected particulate in situ in the filter by a variety of methods. Any technique to increase the exhaust temperature to 600.degree. C. will ignite the collected particulate. Techniques which are normally used to increase exhaust temperature include high engine load, intake air throttling, the use of diesel fuel or a propane burner in the exhaust, or the use of an electric resistance heater in the exhaust to raise the exhaust temperature. The ceramic filters that are used in such cases have several disadvantages however. They have temperature limitations which must not be exceeded. Their thermal conductivity is such that localized hot spots may occur and thus cracking occurs due to thermal gradients or vibrational shock.
The present invention overcomes the disadvantages of the prior art by forming a diesel particulate trap from silicon carbide cermet foam with associated hardware to retain the foam in a particular configuration. Several pieces of the foam, in the shape of "doughnuts", are stacked on top of each other. The stack of foam doughnuts is assembled in a container designed to allow diesel exhaust to flow radially inward through the doughnuts and exit through the center. Ceramic felt gaskets are interposed between the doughnuts and between each of the end doughnuts and steel end plates. These gaskets serve as buffers and allow for different thermal expansion properties of the steel and silicon carbide cermet foam.
The foam doughnuts and gaskets are held together and compressed slightly by two steel end plates. Each plate is slightly larger than the diameter of the doughnuts. One plate is solid and the other has an orifice in the center thereof. The doughnut holes are aligned with the orifice in the end plate. The entire assembly is placed in a steel sleeve having a cone at each end; an exhaust entrance cone and an exhaust exit cone. The diesel exhaust enters the exhaust entry cone and impacts the closed end plate of the doughnut assembly. It then flow around the end plate to enter the space between the outside doughnut surface and the steel sleeve. The exhaust then passes radially inwardly through the silicon carbide doughnuts and exits through the center holes and out the exhaust cone. During passage through the foam doughnuts, the diesel particulate is collected and retained.
The use of the silicon carbide foam overcomes the temperature limitations of the current system. The silicon carbide has a higher melting point than the ceramic most commonly used (cordierite) and its higher thermal conductivity will tend to prevent localized hot spots. The individual doughnuts will be less prone to cracking caused by thermal gradients or vibrational shock because of the buffering action of the ceramic felt spacers. Collecting the particulate by impaction rather than filtration allows for a self-limiting collection mass and therefore minimizes plugging of the filter. In addition, the free silicon remaining in the silicon carbide adds flexibility to the foam to allow for slight movement within the doughnut without cracking.
It is desired that the collected particulate be burned in situ by any of a variety of methods. As stated earlier, any technique to increase exhaust temperature to 600.degree. C. will ignite the collected particulate. With the present invention, the electrical properties of the carbide enable a novel approach to burn the collected particulate. Rather than using an electric heating element to heat the exhaust temperature to 600.degree. C. for combustion, the current is passed directly through each silicon carbide doughnut. The silicon carbide is conductive and has resistance and will become hot enough to burn the collected particulate.
The invention may also be used as a support for catalysts for both diesel particulate traps and gasoline exhausts. Catalysts applied to current diesel particulate trap media lower the temperature requirement for particulate combustion but do not reduce the temperature limitations of the support material. Thus the invention is also usable as a high temperature catalyst support for gasoline engines and natural gas engines.
Thus it is an object of the present invention to provide a novel exhaust particulate trap.
It is also an object of the present invention to provide a diesel exhaust particulate trap utilizing silicon carbide cermet foam as the trap.
It is still another object of the present invention to utilize a trap having an electrically conductive material integrally formed with the filter in the trap for generating heat when an electrical current is passed therethrough to raise the temperature of the filter and burn off particulate matter trapped thereon.
It is yet another object of the present invention to provide a foamed silicon carbide filter element in the shape of a plurality of doughnut shaped segments formed of the foamed silicon carbide with a first conductive mesh unit making electrical contact with the outside of the doughnut and a second conductive mesh unit making electrical contact with the inside of the doughnut so that electrical power can be applied to the two mesh units to provide an electrical current path through the electrically conductive carbide foam.
It is still another object of the present invention to provide a diesel particulate trap having at least one foamed silicon carbide element with a porous construction for trapping the particulate matter and being electrically conductive for generating heat when a current is passed therethrough to thereby burn the particulate matter trapped in the porous material and clean and regenerate the particulate trap.